This invention relates generally to humidity detectors and, more particularly, to a humidity detector utilizing a sensing element comprising a single crystal of sodium chloride which is particularly adapted for use as a respiratory monitor.
A device which is capable of providing an indication of the respiratory rate of a patient and/or providing an audible or visual alarm upon the cessation of respiration is useful in a number of clinical situations. For example, such a device may be advantageously employed during operative procedures and for monitoring patients in the post-operative and intensive care units of a hospital. Various types of respiratory monitors have been employed in such situations.
One such monitor, for example, is sensitive to the movements of the patient's chest associated with breathing. A disadvantage of this type of monitor is that, where the patient's trachea is partially or completely blocked, his chest may continue to move in accordance with his attempts to breath. Under such conditions a respiratory monitor which is reponsive only to chest movement may falsely indicate normal breathing where in fact the patient's attempts to breath are actually ineffective.
Other known respiratory monitors utilize a thermistor as a sensing element. The thermistor may, for example, be installed in a tube which is adapted for insertion into the trachea of the patient. Typically, the thermistor is connected to an electrical circuit which heats the thermistor to a predetermined temperature at which its electrical resistance is known. As air passes by the thermistor due to inhalations or exhalations of the patient, the thermistor is cooled causing corresponding increases in its resistance. These changes in the resistance of the thermistor may be detected by known means to determine the rate of respiration of the patient.
Since the thermistor as a sensing element has to be heated, it cannot be introduced in the nose or the trachea and it cannot be used in the presence of anesthetics in the operating room.
U.S. Pat. No. 3,935,742 to Rybak describes a respiratory monitor utilizing a sensing element which is responsive to changes in the moisture content of the air to which it is exposed. This sensing element uses a thread formed of a rubber or plastic material having a high electrical resistance. The element is installed across the opening of a mouthpiece and is connected to a voltage source. When the patient exhales, moisture condenses on the surface of the thread; the subsequent evaporation of the moisture during inhalation lowers the electrical resistance of the element. These changes in resistance and the corresponding changes in the amount of current flowing through the element may be detected by conventional means to give an indication of respiration.
The apparatus described by Rybak suffers from a number of disadvantages. First, the film of moisture deposited on the sensing element during exhalation may not completely evaporate during inhalation. Rybak suggests that evaporation of the condensation water layer may be accelerated by heating the sensing element. However, such heating may prove dangerous in environments in which explosive gases are present.
Second, since the sensing element of Rybak is exposed to the ambient air surrounding the patient, the accuracy of the measurements obtained may be adversely affected by variations in the relative humidity of the ambient atmosphere.
Also, the relatively long sensing element required in the Rybak apparatus is suitable for installation only across a similarly large opening, such as may be provided in a mouthpiece. A narrow tube, such as a prong of a nasal cannula for supplying oxygen to the nostril of a patient, does not provide an opening large enough to accommodate the moisture-sensitive element suggested by Rybak. Moreover, under certain circumstances, it may be undesirable to insert the mouthpiece required by Rybak into a patient's mouth.
The use of a salt in a moisture sensitive element is known in the art. For example, U.S. Pat. No. 2,975,638 of Morrison discloses a moisture sensitive element in which a salt film is connected in parallel with a thermocouple. Current is passed through the film to heat it and drive off condensed moisture. The thermocouple, on the other hand, tends to extract heat from the film and an equilibrium balance is reached at which the film contains an equilibrium water content. The heating of the salt film required by the Morrison patent is believed to render this type of moisture sensitive element unsuitable for use in the presence of explosive gasses.
U.S. Pat. No. 2,064,651 of Fiene sets forth the use of a solid block of fused salt other than sodium chloride as a moisture sensitive element. The element of Fiene does not offer the advantage of the relatively small size of the single crystal of sodium chloride which is utilized as the sensing element of the present invention.
In accordance with the present invention, therefore, a respiratory monitor is disclosed which overcomes the disadvantages discussed above and offers a number of other important advantages.